With the spread of electronic devices such as notebook computers, mobile phones, and digital cameras, the demand for secondary batteries for driving these electronic devices is on the increase. In recent years, these electronic devices have increasingly high power consumption with enhancement of functionalities thereof and have been expected to be reduced in size, and hence improvement in performance of the secondary batteries has been required. Among the secondary batteries, a non-aqueous electrolyte secondary battery (particularly, a lithium-ion secondary battery) can be increased in capacity, and this battery has thus been applied to a variety of electronic devices.
Generally, a non-aqueous electrolyte secondary battery has a configuration in which a positive electrode and a negative electrode are connected via a non-aqueous electrolyte (non-aqueous electrolytic solution), and stored in a battery case. In the positive electrode, a positive electrode active material layer containing a positive electrode material typified by a positive electrode active material is formed on the surface of a positive electrode current collector. In the negative electrode, a negative electrode active material layer containing a negative electrode active material is formed on the surface of a negative electrode current collector.
In a lithium-ion secondary battery as a typical example of the non-aqueous electrolyte secondary battery, a lithium composite oxide is used as a positive electrode material (positive electrode active material). This composite oxide is described in Patent Literatures 1 to 8, for example.
Patent Literature 1 describes a positive electrode active material obtained by mixing LixCoMO2 and LiNiMnMO2 (both of M are selected from predetermined elements). This positive electrode active material layer includes an active material having a high average voltage at a time of discharge, and an active material with high thermal stability.
Patent Literature 2 describes a positive electrode active material containing a crystal layer with a layered rock-salt structure of LiNiMnTiO2. By containing Ti, this positive electrode active material can obtain high charge/discharge capacities as compared to the case of not containing Ti.
Patent Literature 3 describes a positive electrode active material obtained by mixing LixMnMO4 and LiNiMO2 (both of M are selected from predetermined elements). This positive electrode active material is excellent in battery performance after storage at high temperature.
Patent Literature 4 describes a positive electrode active material in which a portion of Li lacks in LiMnMO2 having a layered polycrystalline structure (both of M are selected from predetermined elements). In this positive electrode, distortion and a chemical bond in the crystal are stabilized, to obtain effects of cycle stability during charge/discharge, durable stability, and the like.
Patent Literature 5 describes a positive electrode active material obtained by replacing a portion of Li and a portion of Co with a predetermined M element in LiCoO2 (both of M are selected from predetermined elements). In this positive electrode active material, by replacement of Li and Co with the M element, binding force between a lithium layer and a cobalt layer is strengthened and distortion between the layers and expansion of a crystal lattice are reduced, to obtain the effects of cycle stability during charge/discharge, durable stability, and the like.
Patent Literature 6 describes a positive electrode active material obtained by mixing LiNiMnCoO2 and Li2MO3 (M is selected from predetermined elements). This positive electrode active material layer includes an active material which exerts an excellent effect on battery capacity and safety and an active material which exerts an effect on cycle characteristics and storage characteristics.
However, any of these positive electrode active materials (positive electrode materials) cannot sufficiently reduce destruction of the crystal structure during charge/discharge, leading to a decrease in capacity of the non-aqueous electrolyte secondary battery, which has been problematic.
For the safety, Non Patent Literature 1 describes a technique of forming a positive electrode containing Ti, namely LiNiMnTiO2.
However, it is described that addition of Ti as described in this Non Patent Literature 1 does not significantly improve the safety.
As another attempt to achieve both the safety and high stability of crystals, Non Patent Literature 2 describes a technique of forming a positive electrode active material that contains Si, having strong binding force with oxygen, in the same amount as that of a transition metal, Namely Li2MnSiO4.
However, the transition metal takes a 4-coordinated coordination structure in this positive electrode active material, causing instability of the structure during charge, and after all, it is not a positive electrode active material having sufficient durability.
Patent Literature 7 describes a positive electrode active material having Li oxide represented by a Li[LixMeyM′z]O2+d(x+y+z=1, 0<x<0.33, 0.05≤y≤0.15, 0<d≤0.1, Me: at least one selected from Mn, V, Cr, Fe, Co, Ni, Al and B, and M′: at least one selected from Ge, Ru, Sn, Ti, Nb and Pt).
However, in a battery using this positive electrode active material, improvement in safety has not been sufficient. Specifically, an addition ratio of the element Me in the transition metal is approximately 14 at %, and there exist oxygen atoms not bonded to the element Me. The chemical bond between the oxygen atoms and the element Me is strong, and chemical bond cleavage (oxygen desorption) hardly occurs. That is, the oxygen atoms not bonded to the element Me contained in the positive electrode active material of Patent Literature 7 become oxygen gas when the battery is formed, which causes deterioration in safety of the battery.
Patent Literature 8 describes that, by having a positive electrode active material with an x-ray absorption spectrum at an oxygen K absorption edge measured by an X-ray absorption fine structure (XAFS) analytical method in which an absorption edge peak that appears in the vicinity of an oxygen K absorption edge at 530 eV shows a certain behavior, reaction on the interface in a charged state is reduced, to improve the battery performance.
However, while the positive electrode active material described in Patent Literature 8 brings an effect of stabilizing the crystal structure on the surface of the positive electrode active material, it has not brought a sufficient effect in terms of safety. Specifically, in the lithium-ion secondary battery (non-aqueous electrolyte secondary battery), there has been a possibility that a crystal structure of a lithium composite oxide used for the positive electrode active material is destructed due to overcharge or the like, to release contained oxygen.
Patent Literature 1: JP 2007-188703-A
Patent Literature 2: JP 2008-127233-A (corresponding to US 2008/116418 A1)
Patent Literature 3: JP 2001-345101-A (corresponding to US 2005/0191551 A1 and US 2002/0012842 A1)
Patent Literature 4: JP 2001-250551-A (corresponding to US 2001/0024753 A1)
Patent Literature 5: Japanese Patent No. 3782058 B (corresponding to US 2003/0013017 A1)
Patent Literature 6: JP 2006-202702
Patent Literature 7: U.S. Pat. No. 8,734,994
Patent Literature 8: JP 2008-127234-A
Non Patent Literature 1: Seung-Taek Myung, and five others, “Synthesis of LiNi0.5Mn0.5−xTixO2 by an Emulsion Drying Method and Effect of Ti on Structure and Electrochemical Properties”, Chemistry of Materials, 2005, vol. 17, p. 2427-2435
Non Patent Literature 2: R. Dominko Li2MSiO4 (M=Fe and/or Mn) cathode materials, Journal of Power Sources, 2008, vol. 184, p. 462-468